ABSTRACT
A common feature of causal mechanisms is that strain depends not only on macroscopic stress but also on an "internal variable" associated with microscopic processes or coupling to another field variable. The Snoek relaxation refers to viscoelastic damping due to motion of interstitial solutes in metals with a body centered cubic (bcc) crystal structure. Piezoelectricity is a coupled-field effect as is thermoelasticity. The asymptotic or equilibrium modulus in the terminal region depends on the density of cross-links and on the molecular weight. In piezoelectric materials stress and strain are coupled to electrical field and polarization. Polyethylene and polytetrafluoroethylene are examples of such semicrystalline polymers. The slip between grains can be directly observed by scribing a line across a polished metal surface and then microscopically observing offsets of the line at the grain boundaries as deformation proceeds. Viscoelastic relaxation can occur whenever there is a delayed rearrangement of the internal structure of the material under stress.
